The present invention relates to a two-blade shutter mechanism for opening and closing a light path by swingingly moving two shutter blades. More particular, the invention relates to a two blade shutter mechanism employing a pulse motor for driving the two shutter blades.
Generally, a so-called two-blade shutter mechanism which opens and closes a light path by swingingly moving two shutter blades is employed in cameras, especially in the 35 mm lens shutter type cameras.
Although the driving action of such a two-blade shutter, that is, the swinging action of the shutter blades, has been conventionally carried out by a spring, recently two-blade shutter mechanism driven by a motor have been developed.
Nevertheless, even if a two-blade shutter is driven by a motor, the system has such a direct shutter swinging mechanism that, for example, two shutter blades are independently swingingly pivoted. A pin protruding from a reciprocally driven lever is caused to pass through a slot defined at the position where the two blades are overlapped, and the shutter blades are swingingly driven by the reciprocal sliding action of the lever through the pin. In this case, the rotational force of the motor is converted into the reciprocal sliding action of the lever.
Further, in the shutter mechanism in which the shutter blades also act as a diaphragm, the shutter blades are generally driven by a pulse motor as the motor described above.
Furthermore, as a drive control method of the pulse motor, there is proposed a control circuit such that the number of pulses in accordance with an exposure time and an aperture are determined, and the motor is rotated in the forward direction corresponding to the number of set pulses when a shutter is opened. The pulse motor is then rotated in the reverse direction when a period of time corresponding to an exposure time has passed from the time at which the final drive pulse forwardingly drives to rotate the motor is applied, whereby a proper amount of exposure is obtaind (refer to Japanese Patent Provisional Publication SHO No. 60-254027). In this control circuit, however, pulses having a predetermined width are always applied to drive the pulse motor.
The above prior art arrangement, however, has a problem in that although the rotation of the motor as a driving source can be very effectively reduced and transmitted by a gear train, it is converted into a sliding movement of the lever at the final stage to swingingly drive the shutter blades and thus the rotational force is very ineffectively transmitted. That is, the driving force of the motor cannot be effectively converted and utilized, which results in the shutter being difficult to be actuated at a high speed and large electrical power is consumed (battery life becomes short).
In addition, the structure of the shutter mechanism described above is complex and the manufacturing cost is also high.
Further, when drive pulses having a predetermined width are used, a pulse width which does not cause so-called step out of the motor at an initial drive (from an initial speed 0 to a time at which driving of the pulse motor starts) should be selected. Thus a slightly longer pulse width than required is set, and thus there is a problem that the operatng time of the shutter is prolonged.
It is generally known that when a pulse motor is energized, damped oscillation is caused by electromagnetic absorption, inertia and frictional resistance before and after the energizing.
FIG. 1 shows an example of damped oscillation of a pulse motor, wherein the horizontal axis represents time and a vertical axis represents displacement. The time necessary for the pulse motor to be stabilized to a constant pulse position S is shown in FIG. 1 when the pulse motor is driven by one pulse commencing at zero seconds.
As shown in FIG. 1 when a drive pulse is applied, the motor oscillates past the stabilizing position S, again returns there and gradually converges on the stabilizing position S. Therefore, a considerable amount of time lapses before the motor converges on this stabilizing position S. When pulses are continuously applied, this damped oscillation occurs for each of the applied pulses. It is possible, however, that an electromagnetic force is changed for the next step when a displacing speed is relatively large at a very early stage of the damped oscillation and thus moves to the next step in a short time using inertia. When this change is not effected timely, however, so-called step out occurs and the motor moves to a position apart from a target position. The term "step out" refers to a position of the motor shaft displaced by one or more steps from a desired position desired, which is based upon the number of pulses applied.
FIG. 2A shows an example of a pulse motor which is driven by a predominant pulse width of 5 ms.
Stabilizing positions at the time are moved stepwise in accordance with each pulse (shown by solid lines in FIG. 2A). Ideally, when pulses are applied along these steps, movable members (shutter blades) are smoothly moved in a short time and securely stopped. In practice, however, the previously mentioned oscillation occurs, and thus the moveable members (i.e., shutter blades) are moved with delay and advance with respect to the stabilizing positions.
Generally, in a case of a three-phase drive motor, when the delay or advance corresponding to 1.5 pulses or more is caused (shown by dashed lines in the figure) step out arises, which causes leaving out of steps or reverse rotation, resulting in displacement different from a set displacement as predetermined by the number of pulses.
FIG. 2B shows another example in which the pulse width applied to the pulse or step motor is shortened to 3.75 ms, wherein although a moving pattern from 0 to 3.75 ms is the same as FIG. 2A, an amount of movement is smaller because of the shortened pulse width and thus when a second pulse is applied, a delay of 1.5 pulses or more is caused with respect to the stabilizing position corresponding to the pulse. Therefore, the pulse motor is caused to be rotated in a reverse direction (along curve D) when the second pulse is applied, and the motor originally intended to be moved to a displacement 2 (along curve C) is moved toward a displacement -1, as if the intended position is defined by pulses along line E, and the motor is positioned between the lower dashed lines B and F. That is, since this movement is out of the area defined by the upper two dashed lines (A and B) in FIG. 2B, which show the area originally intended, step out of the motor arises.
As described above, conventionally, if a pulse width is reduced in order to shorten an operating time, step out is caused, since damped oscillation occurs particularly at the beginning of driving. To cope with this problem, a predetermined pulse width a little longer than required is set within a range in which no step out is caused in accordance with the characteristics of a motor, and the motor is driven with the predetermined pulse width. As a result, a problem arises in that an operating time is increased. In addition, a conventional motor typically overruns due to inertia when a pulse phase is reversed (e.g., to enable a shutter to move from an opening operation to a closing operation). Therefore, a problem arises in that a set aperture is not stable. Further, since the shutter is not securely stopped when the closing operation is completed, the shutter bounds when the operation thereof is completed, and the like.